• Students know a third informal proof of the angle sum theorem.
• Students know how to find missing interior and exterior angle measures of triangles and present informal
arguments to prove their answer is correct.

Lesson Summary

The sum of the remote interior angles of a triangle is equal to the measure of the exterior angle.

Classwork Discussion

Students will see one final informal proof of the angle sum of a triangle before moving on to working with exterior angles
of triangles.

Now let's look at what is called the exterior angle of a triangle. An exterior angle is formed when one of the sides of the
triangle is extended. The interior angles are inside the triangle, so the exterior angle is outside of the triangle along the
extended side.

Exercises 1 - 4

Use the diagram below to complete Exercises 1 - 4.
1. Name an exterior angle and the related remote interior angles.
2. Name a second exterior angle and the related remote interior angles.
3. Name a third exterior angle and the related remote interior angles.
4. Show that the measure of an exterior angle is equal to the sum of the related remote interior angles.